1. Field of the Invention
The invention relates to a super-high pressure discharge lamp of the short arc type in which the mercury vapor pressure during operation is at least equal to 150 atm. The invention relates especially to a super-high discharge lamp of the short arc type which is used as the backlight of a liquid crystal display device and a projector device using a DMD (digital mirror device) and a DLP (digital light processor) or the like.
2. Description of Related Art
In a projector device of the projection type there is a demand for illumination of the images uniformly onto a rectangular screen and with sufficient color reproduction. The light source is thus a metal halide lamp which is filled with mercury and a metal halide. Furthermore, recently smaller and smaller metal halide lamps and more and more often spot light sources have been produced and lamps with extremely small distances between the electrodes have been used in practice.
Against this background, recently, instead of metal halide lamps, lamps with an extremely high mercury vapor pressure, for example, of 150 atm, have been proposed. Here, the increased mercury vapor pressure suppresses broadening of the arc (the arc is compressed) and a major increase of the light intensity is desired. One such super-high pressure discharge lamp is disclosed in U.S. Pat. No. 5,109,181 (JP-OS HEI 2-148561) and U.S. Pat. No. 5,497,049 (JP-OS HEI 6-52830).
In one such super-high pressure discharge lamp, the pressure within the arc tube during operation is extremely high. In the side tube parts which extend from the two sides of the emission part, it is therefore necessary to arrange the quartz glass comprising these side tube parts, the electrodes and the metal foils for power supply in a sufficient amount, and moreover, almost directly tightly adjoining one another. When they are not adjoining one another tightly enough, the added gas leaks or cracks form. In the process of hermetic sealing of the side tube parts, therefore the quartz glass is heated, for example, at a high temperature of 2000xc2x0 C., and in this state, the quartz glass with a great thickness is gradually subjected to shrinking (a so-called shrink seal) or a pinch seal. In this way, the adhesive property of the side tube parts is increased.
However, if the quartz glass is heated up to an excessively high temperature, the disadvantage occurs that, after completion of the discharge lamp, the side tube parts are easily damaged, even if the adhesive property of the quartz glass to the electrodes or metal foils is increased.
It can be imagined that the cause of this disadvantage is the following:
After heat treatment, in the stage in which the temperature of the side tube parts is gradually reduced, as a result of differences between the coefficient of expansion of the material (tungsten) comprising the electrodes, and the coefficient of expansion of the material (quartz glass) comprising the side tube parts, there is a relative difference in the amount of expansion. This causes the formation of cracks in an area in which the two come into contact with one another. These cracks are very small, but together with the super-high pressure state during lamp operation they lead to growth of the cracks; this causes damage to the discharge lamp.
In order to eliminate this disadvantage, the arrangement shown in FIG. 11 was proposed. Here, part of the discharge lamp is shown in an enlarged view. The emission part 10 adjoins a side tube part 11 in which an electrode 2 is connected to the metal foil 3. A coil component 5 is wound around the electrode 2 which has been installed in the side tube part 11. This arrangement of the coil component 5 which has been wound around the electrode 2 reduces the stress which is exerted on the quartz glass as a result of the thermal expansion of the electrode 2. This arrangement is described, for example, in Japanese patent disclosure document HEI 11-176385.
However, in reality, there was the disadvantage that, in the vicinity of the electrode 2 and the coil component 5, cracks K remain even if the thermal expansion of the electrode 2 is relieved by this arrangement. These cracks K are very small, but there are often cases in which they lead to damage of the side tube part 11 when the mercury vapor pressure of the emission part 10 is roughly 150 atm. Furthermore, in recent years, there has been a demand for a very high mercury vapor pressure of 200 atm, and moreover, up to 300 atm. At such a high mercury vapor pressure, the growth of cracks is accelerated during lamp operation. As a result there was the disadvantage that damage of the side tube part 11 clearly occurs. This means than the cracks gradually become larger during lamp operation with a high mercury vapor pressure, even if the cracks K were extremely small at the start. It can be stated that this is a new technical task which is never present in a mercury lamp with a vapor pressure during operation from roughly 50 atm to roughly 100 atm, or no more than roughly 50 atm to roughly 100 atm.
Two of the present applicants have already proposed the arrangement shown in FIG. 12 in commonly-owned U.S. patent application Ser. No. 09/874,231 (corresponding to Japanese Patent Application 2000-168798). In this arrangement. an emission part 10 has a side tube part 11 in which an electrode 2 is connected to a metal foil 3. The electrode 2 with its side 2a and its end face 2b is located in an extremely small intermediate space B out of contact with the quartz glass. This intermediate space arrangement makes it possible to eliminate the above described defect of crack formation if the intermediate space can be formed completely precisely. However, it has been found that, in reality, completely precise formation of this intermediate space is difficult. Specifically, it is disclosed that the intermediate space is formed by applying a vibration to the electrode. However, in practice, the intermediate space cannot be adequately produced by vibration alone.
Furthermore, the arrangement shown in FIG. 12 yielded another, new disadvantage. FIGS. 13(a), 13(b), and 13(c) are each an enlarged representation of the encircled area A of FIG. 12. FIG. 13(a) shows the area A of FIG. 12 in an identical enlarged representation. FIG. 13(b) is a cross section in which the cross section Cxe2x80x94Cxe2x80x2 as shown in FIG. 13(a) is viewed from the top (in direction of arrow D), the position of foil 3 being shown in phantom outline. FIG. 13(c) shows cross section Dxe2x80x94Dxe2x80x2 of FIG. 13(a) viewed from the left side (in the direction of arrow C). As shown in FIGS. 13(a) to 13(c), the intermediate space B is present from the side 2a of the electrode 2 as far as the end face 2b. However, on the end face 2b of the electrode 2, there is an undesirable wedge-shaped intermediate space X.
FIG. 14 shows the intermediate space X in an enlarged representation. Since the intermediate space X is directly connected via the intermediate space B to the emission part 10, the high internal pressure which forms within the emission part 10 (of at least 150 atm) is exerted in the same way. This high pressure is intensely exerted in the wedge-shaped intermediate space X in the directions P3 and P4 of the arrows shown in FIG. 14, and this phenomenon ultimately leads to detachment of the metal foil 3 from the quartz glass. This results in damage to the discharge lamp. It can furthermore be stated that this phenomenon is a characteristic technical task which arises in a discharge lamp which has an arrangement in which the emission part and the end face of the electrode are coupled to one another by an intermediate space, and which has an extremely high internal pressure that is greater than or equal to 100 atm, 150 atm, 200 atm, and moreover, at least 300 atm, as in the invention.
The invention was devised to eliminate the above described disadvantage in the prior art, a primary object of the invention being to devise an arrangement with relatively high pressure tightness in a super-high pressure mercury lamp which is operated with an extremely high mercury vapor pressure.
This object is achieved in accordance with the invention in a super-high pressure discharge lamp of the short arc type which comprises the following:
an emission part in which there are a pair of opposed electrodes and which is filled an amount of mercury at least equal to 0.15 mg/mm3 mercury and
side tube parts of quartz glass which extend from opposite sides of the emission part and in which the electrodes are partially hermetically sealed,
wherein the electrodes are arranged in the side tube parts a respective extremely small intermediate space formed between the sides and the end faces of the electrodes and the quartz glass of the side tube parts, and that the electrodes are provided with concave-convex parts.
The object is furthermore achieved in accordance with the invention by the above described extremely small space being formed of a size that, as a result of the difference between the coefficient of expansion of the material comprising the electrodes and the coefficient of expansion of the material comprising the side tube parts, the electrodes are not constricted in the axial direction, but can freely expand.
The object is furthermore achieved according to the invention by the above described concave-convex parts having a depth of from 1.0 micron to 100 microns.
The object is furthermore achieved by the invention in a super-high pressure discharge lamp of the short arc type which comprises:
an emission part in which there are a pair of opposed electrodes and which is filled with at least 0.15 mg/mm3 of mercury and
side tube parts of quartz glass which extend from both sides of the emission part and in which metal foils which are connected to the electrodes are hermetically sealed,
wherein the electrodes are arranged such that, in the above described side tube parts, an extremely small intermediate space is formed between the sides and the end faces of the electrodes and the quartz glass comprising the side tube parts, the end faces of the electrodes and the metal foils forming an acute-angled arrangement in which the quartz glass is located.
This object is moreover achieved in accordance with the invention by the above described acute-angled arrangement having an angle of less than or equal to 70xc2x0.
The above described arrangement makes it possible to avoid completely or essentially completely the extremely small cracks which form in the side tube parts in the super-high pressure discharge lamp of the short arc type of the invention.
The reason for this is that, for the electrodes located in the side tube parts (upholding parts of the electrodes), there is an intermediate space between the electrode surfaces (including the end faces) and the quartz glass so that the quartz glass and the electrodes do not directly tightly adjoin one another.
In this arrangement, the surfaces of the electrodes are not in contact with the quartz glass. Even if the electrodes move relative to the quartz glass, no cracks due to this motion form between them.
Furthermore, according to the invention, the electrode surfaces are provided with concave-convex parts in order to make these intermediate spaces simple and moreover more reliable.
The technical explanation that formation of the concave-convex shape leads to reliable formation of the intermediate space is not always apparent. As a result of thorough research, the applicant has arrived at the following conclusions:
As is also disclosed in the above described commonly-owned, co-pending U.S. application, in the production process for forming the intermediate space in the last segment of the process of hermetic sealing, an impact is applied to the electrodes. It is assumed that the quartz glass which is in the molten state and which is present in the concave parts is pressed more easily to the outside during the impact if concave-convex parts are present and that the intermediate space is reliably formed by this pressing-out.
Furthermore, the inventors have conducted thorough studies to eliminate the disadvantage of the wedge-shaped space and as a result they have developed a concept for the shape of the end faces of the electrodes.
The invention is explained in detail below using several embodiments shown in the drawings.